In valves such as ball valves intended for use under high pressure conditions, for example in pipelines for transmission of oil and gas, it is often necessary to employ a seal, for example in the form of an O-ring, which is very hard, being made, for example, of Nylon. Such seals present difficulties. For example at low pressure the very hard or high durometer sealing ring may not conform properly to irregular surfaces due to imperfections in the manufacture of the ball or to irregularities in the surface arising out of usage.
On the other hand relatively soft, so-called elastomeric seals are inadequate for high pressure use because they tend to be extruded and can be dislodged from the sealing gland by pressure.